Couch to 5k is great if you're already comfortable running.
I'm not a comfortable runner. Getting better, but not great by any stretch of the imagination.
Yes, I would get off of the eliptical and onto the treadmill. It's a totally different feel. I would also STRONGLY encourage you to do as much outside as you can. A treadmill run/job/walk is nothing like what you'll feel in the race.
If you dont have an outside option make sure you set your incline to a 1 or 1.5. Setting it to 1 or 1.5 simulates natural terrain.
Start slow and build up. Read your body. I also have short legs and a big ***. My heels hurt when I run, my legs throb, my calves aches, my thighs shake and my *** jiggles out of control. I've been "running" (I use that term loosely, my run is a 5.5 on the treadmill) for about a year now.
I would invest in a good pair of running shoes. Because of my weight and my my over pronation when walking/running, I had my shoes fitted and that's fixed a lot of my stride issues.
Don't over do it. Walk a mile or two and see how you feel. Keep that pace for a week and then increase by 10%. (In my triathlon training they say to never increase your time or level of difficulty by more than 10% so your body can adjust and work with you, not against you.)
Personally, I do bouts of speed walking to jogging to a run and cycle through the three.
In your first 5k, I'd suggest finding someone around your pace. (After you start people end up with their pace groups.) I found 4 people that were around my pace and 1 person just a little bit ahead of me. My goal was to finish ahead of those in my pace and try to finish with the person a bit ahead of me. I beat my person best by 2 minutes and my goal by 1:30. There is a lot of energy and endorphins in your first race.
Sorry this was so long....I just remember how nervous I was about my first 5k.